The input voltage, in a single phase alternating-current (AC) electrical outlet, of daily used electrical appliances or electronic equipments in various industries is in the range of 100V to 240V. However, load of each electronic product or device is not the same and the required driving voltage is also different, thus a voltage converter is needed to adjust the power supplies for a variety of applications.
There are different kinds of voltage converter, for example AC/AC converter, AC/DC converter, DC/DC converter and DC/AC converter. In many electronic circuits, some electronic devices require two or more power supplies, for example LCD monitors, voltage comparators, operational amplifiers and the likes, or different operating voltages, thus the DC/DC converter is needed to obtain the desired voltages.
FIG. 1 is a circuit diagram of the existing step-down DC/DC converter, or buck converter, including two switches (a transistor and a diode), an inductor and a capacitor. As shown in FIG. 1, when the transistor switch S is on, current from the input power supply (Vin) flows cross the inductor L, and thus energy is stored in the inductor L. When the transistor switch S is off, the inductor current stored in the inductor L is released to resistor R to maintain the output voltage (VO). The output voltage (VO) converted from the inductor current is then fed back through the feedback circuit 10 to the control circuit 12 for comparing with a reference voltage to control the duty cycle of transistor switch S for achieving a stable output voltage. Refer to FIG. 1, when the transistor switch S is on, one end of the inductor L is connected to the input voltage Vin, and the other end is connected to the output voltage VO. The input voltage must be higher than the output voltage to form a positive direction voltage drop cross the inductor L. When the transistor switch S is off, one end of the input voltage Vin, which is originally connected to the inductor L, is connected to ground GND, thus the output voltage VO is the positive end, forming a negative direction voltage drop cross the inductor L.
In the above configuration, the peak inductor current produced under peak current mode is used to compare with the constant reference voltage to control the transistor switch, which produces a peak-to-average error and therefore produces poor regulation accuracy. As shown in FIG. 2 (a), bigger inductance L creates higher average inductor current, thus the peak-to-average error is relatively smaller; otherwise, as shown in FIG. 2 (b), smaller inductance L creates lower average inductance current, the peak-to-average error is relatively larger. The average inductor current strongly depends on the inductance, resulting in very poor voltage regulation accuracy of the converter.
For this reason, this invention proposes an average inductor current-mode voltage control method using a variable reference voltage and a variable reference voltage generation unit. In the present invention method, the average current's loop compensation or load current sense is adjusted by the inductor current only, and the average inductor current is used to control the constant off time DC/DC converter operating in continuous conduction mode.